A conventional optical head device generally comprises a semiconductor laser as a light source, a diffraction grating for generating three beams from the emitting light emitted of the semiconductor laser, a beam splitter for separating the light incided into two directions, an objective for focusing light on an optical disk and optical detector for converting the inciding light into an electrical signal, wherein a light beam is focused on the optical disk to form a micro spot thereon and a reproduction signal is taken from the reflecting light.
Here, in the optical head device which is to be adapted to a high-density recording medium, the size of a light spot which is projected on the recording medium needs to be reduced. The size of the light spot projected on the recording medium depends on a wavelength .lambda. of laser emitting light and a numerical aperture(N.A.) of an objective. Thus, to minimize the size of the light spot, the wavelength .lambda. is shortened or N.A. is increased.
Japanese patent application laid-open No.3-163732 discloses an optical head device in which a variable aperture iris on the light path of an objective is provided as means for changing the size of a spot projected on a recording medium. Furthermore, Japanese patent application laid-open No.5-120720 discloses an optical head device in which an objective and an aperture diameter varying means which comprises a liquid crystal device and an analyzer, which are integrated and simultaneously actuated by an objective actuator, are provided to vary the numerical aperture. These optical head device can control the focusing to respond to the variation of the disk surface and can control the tracking to respond to the eccentricity of the disk.
However, in the above conventional optical head devices, the tracking cannot be stably controlled since the location of three beams cannot be controlled to respond to the change of the interval between pit rows due to the change of the recording density of the disk. Namely, when the shape(diameter) of the three beam spots is so different from the shape(width) of the tracking pit, such three beam tracking error detection method cannot provide a correct readout of the tracking error signal.
In addition, none of the conventional optical head device has been designed to control the diameter of the beam spot projected on the pit rows on the disk. Namely, in both the case that a high recording density medium is reproduced and the case that a normal recording density medium is reproduced, the diameter of the projecting spot is constant.
As a result, when an optical head device which is designed to adapt to a normal recording density medium is used to read out a high recording density medium, it cannot read out a correct tracking error signal since a projecting spot may overlap with both of the adjacent pit rows due to the long diameter thereof. Also, when an optical head device which is designed to adapt to a high recording density medium is used to read out a normal recording density medium, it cannot read out a correct tracking error signal since the non-sensing area may occur between pit rows due to the short diameter of the projecting spot.